1. Field of the Invention
This invention relates to surgical interbody fixation devices and in particular to a surgically implantable device for the stabilization of adjacent vertebrae of the human spine undergoing spinal arthrodesis and for the prevention of the dislodgement of spinal fusion implants used in the fusion process.
2. Description of the Related Art
When a segment of the human spine degenerates, or otherwise becomes diseased, it may become necessary to surgically remove the affected disc of that segment, and to replace it with bone for the purpose of obtaining a spinal fusion by which to restore more normal, pre-morbid, spatial relations, and to provide for enhanced stability across that segment. Performing such surgery of the spine from an anterior (front) approach offers the great advantage of avoiding the spinal cord, dural sac, and nerve roots. Unfortunately, in entering the disc space anteriorly a very important band-like structure called the anterior longitudinal ligament, is violated. This structure physiologically acts as a significant restraint resisting the anterior displacement of the disc itself and acting as a tension band binding the front portions of the vertebrae so as to limit spinal hyperextension.
Historically, various devices have been utilized in an attempt to compensate for the loss of this important stabilizing structure. These devices have assumed the form of blocks, bars, cables, or some combination thereof, and are bound to the vertebrae by screws, staples, boats, or some combination thereof. The earliest teachings are of a metal plate attached to adjacent vertebrae with wood-type screws. Dwyer teaches the use of a staple-screw combination. Brantigan U.S. Pat. No. 4,743,256 issued on May 10, 1988, teaches the use of a block inserted to replace the disc, affixed to a plate then screwed to the vertebrae above and below. Raezian U.S. Pat. No. 4,401,112 issued on Aug. 30, 1993, the use of a turnbuckle affixed to an elongated staple such that at least one entire vertebral body is removed, the turnbuckle portion is placed within the spine, and the staple extends both above and below the turnbuckle and engages the adjacent vertebrae to the one removed.
Unfortunately, both staples and screws have quite predictably demonstrated the propensity to back out from the vertebrae. This is quite understandable as any motion, either micro or macro, tends to stress the interface of the metallic implant to the bone, and in doing so causes the bone to relieve the high stress upon it by resorbing and moving away from the metal. This entropic change is universally from the more tightened and thus well-fixated state, to the less tightened and less fixated state. For a staple, this is specifically from the more compressed and engaged state, to the less compressed and disengaged state. Similarly, screws in such a dynamic system loosen and back out.
The potential consequences of such loosening and consequent backing out of the hardware from the anterior aspect of the vertebral column may easily be catastrophic. Because of the proximity of the great vessels, aortic erosions and perforations of the vena cava and iliac vessels have usually occurred with unfortunate regularity and have usually resulted in death.
Therefore, the need exists for a device which is effective in restoring stability to a segment of the spine such as, but not limited to, the anterior aspect of the human spine and which will without danger remain permanently fixated once applied.
The present invention is directed to a spinal fixation device for stabilizing a segment of the human spine and for preventing the dislodgement of intervertebral spinal fusion implants, which remains permanently fixated to the spine once applied. The spinal fixation device of the present invention comprises a staple member made of a material appropriate for human surgical implantation and which is of sufficient length to span the disc space between two adjacent vertebrae. The staple member engages, via essentially perpendicular extending projections, the vertebrae adjacent to that disc space. The projections are sharpened and pointed so as to facilitate their insertion into the vertebrae and are segmented or ratcheted to prevent the staple member from disengaging and backing out once inserted.
In the preferred embodiment of the spinal fixation device of the present invention, a portion of the staple member interdigitates with an already implanted intervertebral spinal fusion implant and the staple member is bound to the spinal fusion implant by a locking mechanism such as a screw with a locking thread pattern. The anchoring of the staple member via a locking mechanism to a spinal fusion implant protects the patient from the danger of the staple member itself disengaging and backing out. Further, if the spinal fusion implant is externally threaded, such as the spinal fusion implant taught by Michelson, U.S. Pat. No. 5,015,247 issued on May 14, 1991, then the staple member could only back out if the spinal fusion implant were free to rotate. However, the rotation of the spinal fusion implant in this instance is blocked by its connection to the staple member which is fixated across the disc space in such a way as to be incapable of rotation. Thus, the staple member is made safe against dislodgement by attachment to the spinal fusion implant and the stability of the spinal fusion implant is assured as it is also stabilized by the staple member and each works in connection with the other to remove the only remaining degree of freedom that would allow for the disengagement of either.
The spinal fixation device of the present invention is broadly applicable to the anterior, posterior and lateral aspects of the spinal column, be it the cervical, thoracic or lumbar area. In particular, the use of a staple member spanning the disc space and engaging the adjacent vertebrae which is applied to the anterior aspect of the spine is of great utility in restraining those vertebral bodies from moving apart as the spine is extended and thus is effective in replacing the anterior longitudinal ligament of the patient.
The spinal fixation device of the present invention provides the advantage of facilitating cross vertebral bony bridging (fusion via immobilization) which when achieved relieves all of the forces on the inserted spinal fusion implants. The spinal fixation device of the present invention may be coated with materials, to promote bone fusion and thus promote the incorporation and ultimate entombment of the spinal fixation device into the bone fusion mass. The use of a bone fusion promoting material results in a speedier vertebra to vertebra fusion as bone may grow along the coated spinal fixation device bridging the two vertebrae so that the spinal fixation device acts as a trellis and supplies essential chemical elements to facilitate the bone fusion process.
Another advantage provided by the spinal fixation device of the present invention is that as it is inserted it compresses the adjacent vertebrae together, thus increasing the compressive load on the spinal fusion implants or implants within the disc space, such compression being beneficial to fusion and further stabilizing the spinal fusion implants.
A further advantage of the spinal fixation device of the present invention is that it may be used as an anchor such that a multiplicity of spinal fixation devices may then be interconnected via a cable, rod, bar, or plate, so as to achieve or maintain a multi-segmental spinal alignment.
Alternatively, the spinal fixation device of the present invention could be made of resorbable materials, such as biocompatible resorbable plastics, that resorb at an appropriate rate such that once the spinal fixation device is no longer needed (i.e. when spinal fusion is complete) the body would resorb the spinal fixation device. The spinal fixation device could be only in part resorbable such that the projections of the staple member would be non-resorbable and would remain incarcerated in the vertebrae and sealed off once the resorbable portion of the staple is resorbed by the body.
As a further alternative, the spinal fixation device of the present invention could be made wholly of in part of ceramic and more particularly made of or coated with a ceramic such as hydroxyapatite that would actively participate in the fusion process.
It is an object of the present invention to provide a spinal fixation device having a staple member spanning the disc space and engaging two adjacent vertebrae of the spine to restrain the vertebrae from moving apart as the spine is extended;
It is an another object of the present invention to provide a spinal fixation device that is effective in replacing the function of the anterior longitudinal ligament of a patient;
It is a further object of the present invention to provide a means for protecting the patient from the danger of the spinal fixation device itself disengaging and backing out by its being anchored to an intervertebral spinal fusion implant;
It is still another object of the present invention to provide a spinal fixation device that blocks the rotation of an intervertebral spinal fusion implant by its connection to the staple member which is fixated across the disc space in such a way as to be incapable of rotation thereby preventing the spinal fusion implant from backing out;
It is yet another object of the present invention to provide a spinal fixation device that is broadly applicable to the anterior aspect of the spinal column, be it the cervical, thoracic or lumbar area;
It is another object of the present invention to provide a spinal fixation device which may be applied longitudinally at any point about the circumference of the anterior aspect of the spine;
It is also another object of the present invention to provide a spinal fixation device that stabilizes a surgically implanted spinal fusion implant and works in connection with the spinal fusion implant to prevent disengagement of either;
It is another object of the present invention to provide a spinal fixation device that achieves cross vertebral bony bridging (fusion) which ultimately relieves all of the forces on intervertebral spinal fusion implants inserted within the disc space between two adjacent vertebrae, and provides for a permanently good result;
It is another object of the present invention to provide a spinal fixation device that serves as an anchor, such that a multiplicity of these anchors may then be interconnected via a cable, rod, bar, or plate, so as to achieve or maintain a multi-segmental spinal alignment; and
It is a further object of the present invention to provide a spinal fixation device that directly participates in the bony bridging of two adjacent vertebrae and participates in the spinal fusion process across those vertebrae.